A portal type machine tool is shown in Patent Document 1, and Patent Document 2, respectively. With the portal type machine tool, for example, a portal type machining center, a portal frame is made up by spanning a horizontal cross beam between respective columns provided on the right side and the left side, and the cross beam holds a saddle so as to be freely movable in the longitudinal direction thereof. The saddle is attached to the front face of the cross beam to be movably held by the cross beam as a front-face attached-type saddle, or attached to the upper face of the cross beam to be movably held by the cross beam as an upper-face attached-type saddle.
The front-face attached-type saddle, or the upper-face attached-type saddle is supported at a position on the front side of a machining center of the portal type, with a ram kept in a state reciprocatively movable in the vertical direction, the saddle holding, for example, a working head capable of controlling 5-axes, and an angle-type working head capable of controlling 5-axes at the lower end of the ram. A tool necessary for processing is fitted onto the end of a spindle of the working head capable of controlling 5-axes, thereby enabling the tool to apply a predetermined cutting operation to a workpiece from directions as required.
When the spindle and the tool are brought closer to a machine table upon a cutting operation being applied to the workpiece, the tip of the spindle moves away from support positions such as the ram, the saddle, and the cross beam, and so forth, thereby causing an increase in overhang amount. As a result, the ram, and the tool are susceptible to undergo vibration, and chattering at the time of processing the workpiece, at the time of heavy cutting in particular, so that a cut surface will become rough and the service life of a cutting tool will be significantly reduced, eventually leading to the breakage of the tool.
Further, with a portal type planer of a common structure, because the up-down movement of a movable cross beam is added to the movement of a ram, in the vertical direction, it will be possible to carry out processing of a workpiece large in height. On the other hand, however, the number of movable parts will increase, so that an actual capacity for heavy cutting, and processing accuracy will undergo deterioration as compared with the case of a fixed type cross beam.